Aircraft transparencies such as windshields and canopies are usually designed to provide a relatively distortion-free view for the pilot. Optical acceptance of aircraft transparencies is very subjective. Historically, acceptance relies on a controlled visual assessment of the transparency against a grid board at the facility of the supplier, vendor, or manufacturer, as part of the buyer's acceptance process. The pilot, who typically has no set acceptance process or assessment criteria, also judges acceptability of the windshield during acceptance flights of the aircraft. This subjective acceptance process has created numerous problems with aircraft delivery, and is a major factor in overall cost of the transparency.
FIG. 1 is a diagram that illustrates a transparency test grid 100 and an aircraft windshield 102 through which test grid 100 is viewed. FIG. 1 depicts the “pilot's eye” view through windshield 102. In accordance with ASTM Standard F733-90, the optical acceptance process for transparencies is a controlled viewing against a grid (i.e., test grid 100) of one-inch squares. Parameters such as the distance to test grid 100, lighting conditions, viewing positions, and viewing angles are controlled during acceptance testing. Supplier, buyer, and government personnel review the transparency prior to acceptance and shipment to the buyer facility for installation. This assessment is controlled but subjective, and acceptance levels can vary from individual to individual. The free form manufacturing process of aircraft windshields and canopies may contribute to optical variability from windshield to windshield and canopy to canopy. Typically, the source inspector will develop an eye for “typical or acceptable” optical quality expected from the process, over a period of time. This acceptance level determines how many transparencies will be accepted or rejected according to the process optical distribution curve. Where this acceptance level is drawn is the major factor in determining scrap numbers and therefore windshield costs.
Source inspectors assess optical quality using visual cues such as perceived grid line movement and bending, when moving their head in a prescribed manner. Pilots (the ultimate subjective judges) will accept and reject transparencies by gut reaction and experience without any controlled evaluation techniques. Source inspectors are typically stable in the job while acceptance pilots may come and go with various levels of acceptance, which the source inspectors may react to. Progression is usually to move the acceptance levels higher and higher to avoid on-aircraft pilot rejections. This typically increases rejections and cost, and sometimes slows windshield deliveries to levels below production requirements.
While windshield acceptance is weighted largely by visual review, photographic evaluations are also performed against test grid 100 to evaluate optics. Typically, a camera is set at a specified location and double or triple exposures are taken as the windshield is translated and rotated to different positions. This procedure is described in ASTM Standard F733-90, the relevant content of which is incorporated by reference herein. This double exposure technique is intended to duplicate the movement of the evaluator's head during optical evaluation. Measurements taken from the photographs, such as gridline thickness variation (i.e., apparent growth or shrinkage of the thickness of gridlines caused by the double exposure), and slopes (i.e., variations from the ideal horizontal and vertical gridline orientations) can be identified and measured by hand, however, such results are hit and miss and hard to discern. Moreover, correlation of this data to actual optical quality is uncertain. Consequently, while the double exposure technique is a good method to judge overall optical quality, it does not provide an objective evaluation of actual optical characteristics.
Furthermore, existing testing methodologies for aircraft transparencies do not detect optical distortions that are created during subsequent assembly of the aircraft or optical distortions that are induced by external forces applied to the transparency after the transparency is installed in the aircraft and/or external forces that may be applied to the transparency during flight.